Dismayed by Israel's vulnerability during the 1973 Yom Kippur War, Schack, an American who headed a construction firm, volunteered his services to the Jewish state and was soon posing as a gentile, gathering intelligence in Arab countries from the mid-1970s to the late 1980s for the Mossad. Using the name Howard Mackenzie (``an earnest, non-ideological . . . businessman out to make a buck''), he networked with high-ranking Arab officials and generals from Libya to Kuwait. His sighting of a Soviet ship in a Syrian port contributed to the success of the hostage-rescuing Entebbe raid in 1976, and he obtained information about the construction of a nuclear-reactor facility outside Baghdad that led to Israel's 1981 bombing of Iraq's Osirak plant. Schack is not at all modest or self-effacing; he records many compliments and expressions of gratitude received from his Mossad handlers--readers will feel every one of them was richly deserved. His suspenseful account, written with Jeffers, coauthor with Sheila MacRae of Hollywood Mother of the Year , is like a Walter Mitty fantasy come to life, and Schack's jaunty humor and observations of Arab culture and customs deepen the reader's enjoyment. Photos. (July)